Method for reducing corona in dynamoelectric machines

ABSTRACT

According to the invention, electrically conductive paths are formed between the coil armor and the core laminations of dynamoelectric machines for inhibiting corona. This is achieved by injecting an uncured, semi-conducting, elastomeric material between the coil sides and the walls of the core slots by way of the air ducts in the core, and thereafter curing the material. The uncured material has a viscosity which enables it to be forced under pressure between the coil sides and slot walls by means of a pneumatically actuated gun assembly whose construction and operation are described in detail. The cured material has a resistance high enough not to form eddy current paths between laminations and yet low enough to conduct charge from the coil armor to the core. It is also capable of retaining its strength, elasticity, conductivity, etc., and remaining in place between the coils and core under vibrations, coolant flow, electric stresses, repeated temperature changes, etc., for the normal operating life of the machine.

This is a Division of application Ser. No. 656,865, filed Feb. 10, 1976,now U.S. Pat. No. 4,068,691, issued Jan. 17, 1978.

BACKGROUND OF THE INVENTION

This invention relates to a method for providing conductive paths fromthe outer surface of conductor insulating jackets to the magnetic coresin dynamoelectric machines for purpose of inhibiting corona in themachine.

In a laminated magnetic core for a dynamoelectric machine the dimensionsof the teeth vary somewhat between laminations, and the positions of thelaminations vary in the core stacks. These irregularities are greatenough that the surfaces of the slots have somewhat jagged faces. Thecoils used in the machine are insulated with outer jackets consisting ofwrappings of porous materials impregnated with certain thermosettingresins and shaped in a mold while the resin is cured to a solid and hardstate. This leaves the outer surfaces of the coils very smooth, hard andwith some irregularities in their flatness. When these coils are inplace in the slots, the smooth outer surfaces of the coils make physicalcontact with some of the high laminations, leaving voids between thejacket and other laminations.

Variations in coil side dimensions can lead to looseness of sides intheir slots, resulting in voids or exaggerating the voids mentionedabove. The tolerance of a coil side may be in a range of several mills.It is known to insert packing strips between a coil side and a slot wallto tighten-up the fit and thereby prevent movement of the side in theslot. These strips may be thin, non-metallic, electrically conductivesprings which secure the side in the slot and provide electrical pathsof controlled resistance between the coil armor and the slot wall.However, since the strips come in discrete thicknesses that can bedriven between a coil side and a slot wall, this packing may not alwaysmake a side a tight fit in its slot; any looseness may lead to coilmovement resulting in corona problems.

Electrical grade resinous materials should be good insulators ofelectricity and reasonably good conductors of heat. Certain epoxy resinsmeet this specification. However, those that do meet the specificationcure to a hard state, and once fully cured, they do not softenappreciably when reheated during operation of the machine. Thesematerials produce the so-called hard-bar windings in which the resinimpregnants do not soften when the coils become hot and flow into thevoids as did the asphaltic impregnants that preceded them. Because theresinous materials do not soften with heat and flow into the voids, thevoids remain.

Initially, the armor covering on the coils usually make good electricalcontact with many of the laminations defining the slot walls. Thesecontacts placed the armor and core at essentially the same potential.However, vibration from machine operation will often break thesecontacts and cause sufficient coil movement to lead to a difference ofpotential between the armor and core. This potential difference imposeselectrical stresses on the air in the voids formed at the breaks,stresses that may well be great enough to cause partial discharge fromthe coil surfaces to the core, i.e., a phenomenon often referred to ascorona or corona discharge. The improved resinous materials make higheroperating voltages possible, and this in turn subjects the void regionsto high electrical stresses, or these newer insulations may evenincrease stresses without an increase in voltage. It is well known thatin the presence of corona discharge insulating materials are eroded andmay eventually break down.

Our Canadian Pat. No. 1,016,586, issued Aug. 30, 1977 and entitled"Grounding of Outer Winding Insulation to Cores in DynamoelectricMachines," describes and claims a means for inhibiting corona indynamoelectric machines such as large power generators. In thisapplication, an elastomeric material of controlled electrical resistanceis applied to the coil sides and then cured before the sides areinserted into the slots in the core. The lay of this material on a coilside is such that the material deforms as the side is inserted into aslot, causing the material to make contact with the laminations. In thisparticular approach to the corona problem, the elastomeric material isapplied to the coil sides before they are inserted in the slots; thematerial cannot be applied to the coil sides already in place in theslots.

U.S. Pat. No. 3,824,683 issued July 23, 1974, Rhudy, discloses a methodfor reducing corona in machines having the coils in place in the slotsof the core, for example, treating a machine that has been in service.In this particular method, a free flowing, electrically conductive paintis made to flow in between the coil sides and the slot walls so as tocoat both. After the paint is dry, an elastomeric material is depositedin some of the air ducts in the core in contact with the coil surfacesand core. This material contains an electrically conductive fillerwhereby conductive paths are provided between coils and core.

The object of this invention is to improve the inhibition of corona indynamoelectric machines having the coils in place in the slots of themagnetic core.

According to the invention conductive paths are formed between thewinding and the core of a dynamoelectric machine by injecting a viscous,semi-conducting, elastomeric material between the coil sides and theslot walls by way of the air ducts in the core, and thereafter curingthe material. The cured material is a tough rubber-like substance of anelectrical resistance high enough not to short circuit the laminationsof the core and yet low enough to conduct electric charge from the coilarmor to the core; it is a substance that is capable of retaining itsstrength, elasticity, conductivity, etc., and remaining in place betweenthe coils and core under vibration, coolant flow, electric stresses,repeated temperature changes, etc., for the normal operating life of themachine. These paths conduct electric charge from the coils to the core,and thereby inhibit the formation of corona.

Certain silicone resins are well suited for use as conductive pathforming materials between the coils and core. Inherently, siliconeresins are good electrical insulators, but some are relatively goodconductors of heat as well. The good heat conductors are preferredbecause they will transfer heat from the coils to the core. To make themelectrically conductive for purposes of conducting electric charge, theyare filled with conductive fine particle materials such as carbonpowder, lamp black or a mixture thereof. The amount of conductive powderadded to the resin is just enough to give the cured product thenecessary electrical properties, but not enough to detract significantlyfrom its physical properties.

Apparatus for injecting an uncured elastomeric material into the spacebetween a coil side and a wall of the slot containing the side consistof an injector tool adapted for insertion into a selected air duct inthe core into communication with the coil side for material flow fromthe tool into the space between the side and slot wall; means forsecuring the tool in the duct while material flow takes place; and meansfor forcing material flow from the tool into the space. In a preferredapparatus, the securing means and the flow forcing means may bepneumatic actuating means.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the invention will now be described in moredetail with reference to the accompanying drawings in which:

FIG. 1 is an exploded perspective view of a small portion of the statorof a large dynamoelectric machine and a gun assembly for injecting anuncured elastomeric material into the spaces between the coil sides andthe slot walls;

FIG. 2 is a view in perspective of the injector tool;

FIG. 3 is a view of the injector tool in place in an air duct forinjecting the material between a coil and a slot wall; and

FIG. 4 is a view of the material flow pattern.

DETAILED DESCRIPTION OF THE INVENTION

For purposes of the foregoing discussion and the description to follow,a winding for a dynamoelectric machine is to be regarded as a largenumber of coils interconnected in a suitable circuit configuration. Eachcoil may have one or more turns of either a single conductor or a numberof parallel strands. When references are made to coil sides, these arethe portions of a coil that are located in the slots of the core.

FIG. 1 shows a portion of a stator for a dynamoelectric machine such asa large generator and apparatus for injecting an uncured elastomericmaterial into the spaces between the coil sides and the slot walls. Thestator consists of a laminated magnetic core 10, a winding 11 and theusual supporting structure, none of which is shown. The core is made upof a plurality of stacks 12a, 12b, etc., of laminations, the stacksbeing spaced apart axially by means of the radially disposed spacers13a, 13b, etc., so as to define radial air ducts 14a, 14b, etc. The corecontains a plurality of axially directed slots 15a, 15b, etc., and thewinding consists of a plurality of coils 16 having sides 17 and 18located in two different slots and end turns 19 projecting from the endsof the core. Each coil has one side 17 located in the bottom of one slotand the other side 18 in the top of another slot at approximately onepole pitch coil span. This is a conventional core having a plurality ofequally spaced slots containing a conventional distributed winding, awinding frequently found on the primary side of AC motors or on thearmature of AC generators. The invention to be described later is in noway limited to this or any other particular core and coil arrangement.

Each coil 16 is shown as consisting of a plurality of conductor strandsinsulated from one another and encased in a jacket consisting of aninner insulating jacket 20 and an outer armor jacket 21. The strandinsulation may be a covering of bonded glass filaments. After thestrands have been formed into a coil of the desired number of turns andshape, the insulating and armor jackets are applied. Typically, theinner jacket is a number of layers of insulating materials such as resinbonded micaceous tapes, and the armor is one or more layers of asemi-conducting tape or paint, i.e., a material having a controlledresistance. The tapes may be several mils in thickness, and they may beapplied in a number of layers, depending on the voltage that they mustwithstand. The layers are applied tightly and as uniformly smooth aspossible, after which the resins in the tapes are cured by heat andpressure to achieve the necessary insulating properties. Even with themost careful application of the tapes and curing of the resins with thecoil sides in pressure molds there will be some variations in width ofthe coil sides and flatness of their radial surfaces, i.e., thesesurfaces are frequently slightly concave. Hence, even with packing,there may well be some looseness of coil sides in their slots. Whenfully cured the resin bonded materials become very hard and rigid and donot soften appreciably when reheated. As a result, the radial surfacesof the coils present somewhat irregular and unyielding interfaces withthe irregular surfaces of the punchings forming the slot walls. The hardcoils do not soften when reheated from machine operation and deformsufficiently to fill the slots as did the asphaltic compounds of theprior art. Initially, the semi-conducting armor may have made goodelectrical contact with the core laminations so that the armor and slotwalls were held at essentially the same potential. However, vibrationfrom operation of the machine will often break these contacts and causesome coil movement. This may well enlarge the voids already present oreven create new ones and thereby stress the air in the voids to coronalevels, i.e., stresses at which partial discharge from the coil armor tothe core takes place.

Apparatus for injecting an uncured elastomeric material into the spacesbetween the coil sides and the slot walls is the hand held gun assemblyillustrated at 25 in FIG. 1 combined with a dispenser which is notshown. The dispenser is an air operated hydraulic pump which forces theuncured elastomeric material from a pail into a high pressure ram pump;the ram pump then forces the material at several hundred pounds pressurethrough the flexible hose 27 to the non-drip metering valve 26 of gunassembly 25. Valve 26 is mounted on a hand grip 28 which also houses thevalve operating mechanism. This mechanism is operatively connected tothe dispenser electrically and pneumatically by way of conductors 29 andflexible hose 30 for purposes of co-ordinating the operation of thecombination. The injection apparatus described so far is known andavailable commercially, for example, it is used in building constructionfor sealing windows and masonry joints with thick paste-like compounds.

In addition to the known assembly of components 26 to 30, gun assembly25 includes a number of other components now to be described. Theseother components consist essentially of a body portion 31, an injectortool 32 and a guide member 33 projecting from the body, tool securingmeans 34, a pneumatic actuator 35 supported on the body and operativelyconnected to the tool securing means, an air line 36 for connecting theactuator to a source of compressed air, and a control valve 37 in theair line for controlling operation of the actuator. Metering valve 26 isalso attached to body 31; hence, components 26 to 37 comprise a singleassembly. Reference should now be made to FIG. 2 as well as FIG. 1. Tool32 is in the form of a stem 38 which terminates in a head 39. The stemand head contain a fluid flow passage 49 extending from metering valve26 to an orifice 41 in the head. Hence the uncured elastomeric materialreleased by the metering valve flows through this passage and is ejectedfrom the orifice. Tool 32 and guide 33 project from body 31 in the samedirection in substantially parallel relation, and their spacing is suchthat they can be inserted in a pair of air ducts 14 astride a spacer 13in the way indicated in FIG. 1 by the phantom lines 42 and 43 for ducts14a and 14b.

FIG. 2 illustrates tool 32 and its securing means 34 in perspective andFIG. 3 shows them in place in an air duct 14. One side of head 39 isformed with a flat face 44 of rectangular outline of width 45 and length46 and having orifice 41 in the middle of the face. The flat surface ofthis face is designed to fit against the radial surfaces of the coilsides where they pass through the spaces between the stacks 12 oflaminations. The width 45 of face 44 is as great as the interstack spacewill accept and yet allow the head to be readily inserted and withdrawnfrom the duct, and its length 46 is approximately equal to the radialdimension of a coil side. When the tool is in place in an air duct asillustrated in FIG. 3, it has face 44 pressed against a radial coilsurface, it has the long edges 47 and 48 of the face in very closeproximity with the lamination stacks, and it is secured in this positionby securing means 34.

The securing means illustrated consists essentially of an obliquesurface 49 on head 39 on the side thereof directly opposite face 44, awedging member 50 having a complementary surface 51 bearing on surface49, and a strut 52 connecting the wedging member to the operator ofpneumatic actuator 35 so that the actuator can cause the wedging memberto slide along these surfaces for purposes of securing or releasing thetool. Surface 49 lies in a plane that diverges from the plane of face 44as it progresses along the head from the tip thereof. The complementarysurface 51 on the wedging member slopes in the opposite direction.Hence, movement of the wedging member relative to the head is bothlongitudinal and transverse. Movement up the inclined surface 49 drivesthe back surface 53 of the wedging member against a spacer 13, securingthe head of the tool in a duct with its face 44 pressed against a radialsurface of a coil side. The tool is now in place ready for injecting theuncured elastomeric material between a coil side and slot walls.Movement of the wedging member down the inclined surface 40 releases thetool so it can be withdrawn from the duct and inserted into anotherduct. In FIGS. 2 and 3, tool 32 is shown with face 44 set out a littlefrom the side of the stem. This is not necessary; the face on the headand the side of the stem may lie in the same plane, and the width ofthis plane surface may be uniform throughout the length of the tool. Itis, of course, obvious that the tool must be small enough throughout itslength to allow it to be inserted into the ducts, and long enough toreach the sides in the bottoms of the slots.

The method of injecting the uncured elastomeric material between coilsides and slot walls will now be considered. Holding gun assembly 25 inthe hands, insert tool 32 and guide 33 into a pair of air ducts 14astride a spacer 13 with face 44 next to the coil sides to be treated.The stem of the tool may be marked to indicate the depth that the toolis to be inserted for either a lower or an uppper coil side. Preferably,begin with the lower side and then withdraw the tool to treat the upperside. The tool is properly placed when its face 44 covers the radialsurface of the coil side as shown in FIG. 3. With the tool in place,apply compressed air to actuator 35 by operating valve 37, causing theactuator to draw wedging member 50 up the inclined surface 49. Duringits travel, the wedging member abuts spacer 13 and thereby forceslateral movement of the tool head until its face is pressed firmlyagainst the radial surface of the coil side. The tool is now in placeready for injecting the uncured elastomeric material between the coilside and slot wall. Material injection is effected by actuating valve 26to release the pasty material. Since the material released is underrelatively high pressure, it is forced out of orifice 41 along face 44and in between the surface of the coil and the slot walls of thelamination stacks on both sides of the tool. The quantity of materialreleased by valve 26 on each actuation can be set, and once set thevalve releases the same quantity each time. The quantity needed isdetermined experimentally to give a distribution somewhat as illustratedin FIG. 4 where the cross-hatched areas 54 and 55 depict the flowpattern on either side of the interstack space 56. Finally, the tool isreleased from its lower coil position, repositioned and secured in theupper coil position, and the material injected as before. This procedureis repeated for the various ducts until a layer of the material has beeninjected between the slot walls and at least one radial surface of allthe coil sides. The material is now allowed to cure under roomconditions by simply leaving it undisturbed for a period of time. Onelayer is usually enough because any looseness of a coil side in its slotis taken up by the pasty material forcing the side over firmly againstthe other side of the slot as the material is squeezed between the sideand slot wall.

FIG. 4 illustrates the distribution of the material on a radial coilsurface between adjacent interstack spaces 56 and 58. As already pointedout, the material flows in opposite directions from the tool, coveringareas 54 and 55 from space 56. It does the same in the adjacentinterstack space 58 where areas 57 and 59 are covered. It is to be notedthat patterns 55 and 57 unite to cover most of the coil surface between56 and 58; only two small areas 60 and 61 remain where coverage may notbe as complete as in areas 55 and 57.

The uncured material referred to above is a type which cures rapidlywhen exposed to air, becoming a tough rubbery substance which holds thecoil sides firmly in their slots and provides corona discharge pathsfrom the coil armor to the core. As is to be expected, some of thismaterial is also extruded into the air ducts, where it can block machineventilation if not removed. The material lodged in the ducts can beremoved by allowing it a short time to stabilize to a consistency whereit is easily scraped or wiped off. However, this time must be watchedcarefully; otherwise the material may cure to a rubbery state where itis very difficult to remove.

The material forced between coil sides and slot walls is an electricalconductor in the sense that it allows electric charge to flow from thecoil armor to the core, and in so doing it inhibits the destructiveeffects of corona. Materials found suitable for this purpose are certainroom temperature vulcanizing (RTV) silicone resins. This type of resinis normally a heat conductor and an electrical insulator. Hence, torender it electrically conductive it is filled with fine particles ofgraphite and/or lamp black dispersed in the material. Examples ofsuitable silicone resins are those sold by the General Electric Company,Waterford, N.Y. as RTV-108 and CRTV-5120, the former being an unfilledthermally conductive silicone resin and the latter a filled electricallyand thermally conductive silicone resin. The amount of filler added isjust enough to give the material an electrical resistance within a rangeof approximately 1000 to 80,000 ohms per square, and preferably about4000 ohms per square. A resistance controlled within this range rendersthe cured material sufficiently conductive for readily passing charge onthe coil armor to the core and sufficiently non-conductive that it doesnot form eddy current paths between the core laminations. The resistanceof the cured material is of the same order of magnitude as theresistance of the coil armor. Materials answering this description areknown in the art as "semi-conducting" materials. The amount of filleradded is low enough that the physical and chemical properties of thepolymer are not changed to a significant extent, and moreover, theseproperties along with the electrical resistance are maintained over thelife of the machine. An example of a suitable silicone rubbercomposition is one comprising from about 20 percent to 50 percent byweight of electrically conductive carbon powder and from about 80percent to 50 percent by weight of silicone rubber polymer. Thisparticular composition cures rapidly at room temperature when exposed tothe ambient, and remains stable during normal operation of the machine.

Silicone resins such as those mentioned above cure to become very toughand durable polymers which also bond fast to clean surfaces that theycome into contact with. Hence, the cured material not only fills spacesbetween coil sides and slot walls, but it may also bond them firmly intheir slots. This is very good so long as there is no need to remove anyof the sides. However, in many instances it may well be necessary toreplace defective coils. If the coils are to be removable, as theyusually must be, a release agent should be applied to the coil and/orslot surfaces before the silicone resin contacts them. A release agentis a substance which prevents the polymer from adhering to thesesurfaces. In the case of a machine in the process of manufacture, a thinfilm of a release agent can be applied to the slot walls before the coilsides are inserted in the slots; usually, the release agent will be aliquid. Once the coils are in place in their slots, i.e., a woundmachine, the release agent must be in liquid form to enable coating thecontiguous surfaces where the film is needed. Predetermined amounts ofthis liquid can be squirted into regions from where it will flow inbetween the surfaces to be kept free from polymer adhesion. Wetting ofthese surfaces with the release agent is followed by injection of theelastomeric material. If a machine has been in service for some time,the surfaces may first need cleaning with a liquid solvent applied inthe same way as the release agent.

A material found to be suitable as a release agent is a Dupont liquidcontaining a dispersion of polytetrafluoroethylene (PTFE) and known inthe trade as Vydax AR. This material is diluted one part by volumeliquid to four parts by volume trichloroethane to obtain a mixcontaining 4% solids PTFE. When the coil and core surfaces have beenwetted with this mixture before injection of the resin, the curedproduct does not adhere to them and acceptable resistivity readings areobtained.

While our invention has been described herein with respect to certainpreferred embodiments and specific examples, numerous modifications andchanges will readily occur to those skilled in the art. We intend,therefore, to cover all such modifications and changes as fall withinthe scope of this disclosure.

What is claimed is:
 1. The method of inhibiting corona in adynamoelectric machine including a core having a plurality of stacks ofmagnetic laminations spaced apart axially to define interstack spaces;axially directed slots in said core; spacers disposed radially in saidinterstack spaces; and coil sides disposed in said slots; said methodcomprising:(a) positioning an injector tool in an interstack space andadjacent to a radial surface of a coil side, said injector toolincluding a flat face of width slightly less than the width of saidinterstack space and said face having an orifice in the middle thereoffor accommodating flow of an uncured, semiconducting, elastomericmaterial from a source connected to said injector tool; (b) securing thetool in said interstack space with the flat face pressed against saidradial surface of said coil side; (c) releasing said uncured materialfrom said source for flow through said orifice, along said face, andbetween the radial surface of said coil side and the slot walls adjacentto said tool; (d) releasing said tool; (e) repeating steps (a), (b),(c), and (d) for other coil sides in this and other interstack spaces;and (f) curing said material to form a tough, rubbery compound having anelectrical resistance low enough to conduct electric charge and yet highenough to not cause significant eddy-current flow between corelaminations.
 2. The method of claim 1 including the further step ofapplying a release agent to the coil side and/or slot walls beforematerial injection to stop the material from adhering to them.
 3. Themethod of claim 1 wherein said flat face of said injector tool has alength approximately equal to the radial dimension of said coil side.